The combustion of solid fuels in a circulating fluidized bed, e.g. to produce steam, is known and has been described in European Patent 0 046 405, in published German Application 38 00 863 and in the corresponding U.S. Pat. No. 4,884,408.
It has been found that the combustion gas (flue gas) produced by the combustion of hard coal or also of brown coal will have a high content of the nitrogen oxide N.sub.2 O. That N.sub.2 O will increase the greenhouse effect in the atmosphere and will promote the decomposition of ozone. N.sub.2 O is decomposed at about 850.degree. to 1100.degree. C.